A comparative vibrational spectroscopic investigation of free mn-12S2O2 and fn-12S2O2 dithiacrown ethers based on DFT calculations

A successful assignment for the fundamental bands observed in the experimental IR spectra of mn-12S2O2 and fn-12S2O2 dithiacrown ethers was achieved by the aid of the density functional theory (DFT) based quantum mechanical calculations carried out at the B3LYP/6-31G(d) and B3LYP/6-31+G(d) level of theory. Two different scaling approaches, ‘(i) scaled quantum mechanics force field (SQM FF) methodology’, and (ii) the ‘scaling frequencies with dual empirical scale factors’, were used in order to fit the calculated harmonic frequencies to the experimental ones. Potential energy distribution (PED) calculations were carried out to define the internal coordinate contributions to each normal mode and to define the corresponding normal modes of the molecules. The effects of the conformational differences onto the IR active normal modes of the two isomeric molecules and their corresponding experimental frequencies were discussed in the light of the calculated spectral data.